How to draw on canvas on mousedown? - html

Question

I am looking to simply add a dot to the canvas.
I have the following code:
var canv = document.getElementById("myCanvas");
var context = canv.getContext("2d");
var radius = 5;
var putPoint = function(e){
context.beginPath();
context.arc(e.clientX, e.clientY, radius, 0, Math.PI*2);
context.fill();
}
canv.addEventListener('mousedown', putPoint);
I was learning how to do this with a video tutorial. However they were setting the canvas
as the full width/height of the browser window, whereas my canvas is on 400px * 400px and is contained within a div. I think this is the problem.
So my question is are the "e.client" parameters not working because of my canvas being only
a small part of the window?
If so, how can I track the mouse co-ordinates on my canvas?

Answer 1

You must adjust e.clientX/e.clientY by the offset of the canvas element.
Otherwise you're miscalculating the position of your mouse and your dot is probably being drawn outside the bounds of your canvas.
Here's your code modified to take the canvas offset into account.
var putPoint = function(e){
var BB=canvas.getBoundingClientRect();
var offsetX=BB.left;
var offsetY=BB.top;
var mouseX=e.clientX-BB.left;
var mouseY=e.clientY-BB.top;
context.beginPath();
context.arc(mouseX,mouseY,radius,0,Math.PI*2);
context.fill();
}